The conventional methods of patterning integrated circuit structures use a photoresist, i.e., a polymeric composition sensitive to certain radiant energy such that a developer solvent will selectively remove only those portions of the photoresist which have been exposed (or for different compositions, selectively only those portions which are unexposed). After removal of portions of the photoresist, a patterned photoresist layer is left in place on a substrate which provides a patterned mask for subsequent steps such as ion implantation, etching, or patterned deposition of materials by lift off techniques (i.e., depositing a material over all and then removing the remaining portions of photoresist to leave the material only where the photoresist was not present).
Significant problems in a lack of conformal coating of the pattern of the reticle through the resist onto the substrate have been encountered in patterning methods, particularly where patterning is attempted in upper layers of circuit structures where patterns of lower levels provide substantial topography. These problems are due in part to the unequal thickness of resist materials over the topography of the lower levels. Problems also arise from reflections from lower layer structures during exposure of resist materials above the structures. In such cases multilayer resist processes have been proposed.
Conventional bilayer resist processes use a thin resolution top layer resist sensitive to the near UV or violet spectrum on top of a thick planarizing layer of deep UV sensitive resist. Standard lithographic processes are used to pattern the top layer with near UV or violet light. The pattern is transferred into the bottom layer using a flood (i.e. not patterned) exposure with deep UV light, usually in the spectral region of 200 to 260 nanometers, followed by a develop step. Most frequently used conventional optical imaging systems operate in the range of from 365 nanometers on up through about 450 nanometers for patterning the resolution layer. However, conventional multilayer resist processes are subject to problems including limited resolution capability, difficulty of process, unreliability, susceptibility to reflection effects, and relative costliness of light sources for exposure.
An example of a conventional bilayer method is shown in B. J. Lin, SPIE vol. 174, "Developments in Semiconductor Microlithography IV" (1979), incorporated herein by reference. Described therein is a bilayer resist method in which a relatively thick planarization layer of photoresist sensitive to deep UV light is first disposed over a partially fabricated semiconductor structure. A second relatively thin layer of near UV sensitive photoresist is spun onto the planarization layer. The resist on top is chosen to be opaque for deep UV light. The top photo resist is delineated with patterned exposure by near UV light. The patterned top layer then serves as a mask for exposure of the planarization layer photoresist which can be delineated with a blanket deep UV exposure.
It has also been proposed to use a combination of deep UV sensitive photoresists for both the planarization layer and top layer in bilayer photoresist schemes. See, A. W. McCullogh, E. Pavelchek, and H. Windischmann in J. Vac. Sci. Techno., B4(4) Oct.-Dec. 1983, pp. 1241-1246, as well as E. Ong. R. M. Baker, and L. P. Hale in J. Vac. Sci. Technol., B1(4), Oct.-Dec. 1983, pp. 1247-1250, and also B. J. Lin, E. Bassous, V. W. Chao, and K. E. Petrillo in J. Vac. Sci. Technol., 19(4), Nov./Dec. 1981 at pages 1313 to 1319, each incorporated herein by reference, each describing bilayer schemes wherein a deep UV resist is used as a resolution layer on top of another deep UV resist used as a planarization layer. Both layers are exposed with deep UV light with the planarization layer exposed at the same or a deeper wavelength than the resolution layer.
As pointed out above, reflections from structures below the resist material present problems in photoresist patterning. If reflections from substructures occur during a patterned exposure of a resist layer, it frequently occurs that the reflections spread radiant energy through the resist outside the pattern for which exposure is desired. Consequently, portions of resist are exposed (by light reflected from substructures) which are not intended to be exposed. During development these inadvertently exposed portions are developed along with the intended portions resulting in an inaccurate transfer of the intended pattern.
In an attempt to reduce problems arising from reflections from structures below the resist layers in bilayer resist schemes, it has been proposed to add a dye to the planarization layer photoresist. See, K. Bartlett and G. Hillis in SPIE Vol. 394, pages 49-56, Mar. 16-17, 1983, incorporated herein by reference, wherein a dye, Coumarin 6 is mixed into the PMMA resist (sensitive to deep UV light) used as the lower resist layer before the lower layer resist is deposited on the semiconductor structure. Kodak 809, a photoresist sensitive to near UV light, is used as the top layer resist. Coumarin 6 strongly absorbs near UV light of the wavelength used to expose the Kodak 809 photoresist of the top layer. Consequently, during the patterned exposure of the Kodak 809 resist, light passing down into the PMMA resist layer is absorbed by the Coumarin 6 before reflecting back up to the Kodak 809 layer and inadvertently exposing unintended portions of that layer. Problems from reflections from substructures are, accordingly, reduced.
Dyes are also used to reduce reflections in single layer resist processes. See, M. Bolson, G. Buhr, H. J. Merrem, and K. van Werden in Solid State Technology, February 1986 at pages 83-88, wherein it is disclosed to add dyes to resists in single layer photoresist processes to minimize reflections from lower layer structures.
However, the previously proposed resist schemes are subject to certain limitations. The near UV exposure required for patterning the resolution layer in some schemes does not provide as high a pattern resolution as is often desired. Moreover, all the previously proposed schemes expose the planarization layer resist using deep UV light. Conventional sources of such deep UV light are relatively costly and provide a relatively low power output in comparison with conventional near UV light sources. The previous systems also are susceptible to aberrational patterning from reflection effects or complications in the processes to attempt to reduce those effects. Accordingly, a need exists for photoresist patterning providing high resolution over severe topographies, improved reflection control and which has inexpensive yet faster photoresist illumination.